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step_info overshoot error #554

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juanodecc opened this issue Mar 2, 2021 · 2 comments · Fixed by #555
Closed

step_info overshoot error #554

juanodecc opened this issue Mar 2, 2021 · 2 comments · Fixed by #555

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@juanodecc
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Hi, the overshoot calculate by step_info function is wrong for this transfer function:

import control as ctrl #control 0.8.3
import matplotlib.pyplot as plt #matplotlib 3.2.2

transferFunction = ctrl.tf([0.6287,37.74,774.4,5797,7515],[1.629,45.74,788.4,5809,7515])

time, out = ctrl.step_response(transferFunction)

plt.figure()
plt.grid()
plt.plot(time, out)

fig

ctrl.step_info(transferFunction)

{'RiseTime': 0.051990934709959705,
'SettlingTime': 0.4289252113571676,
'SettlingMin': 0.9121688025464874,
'SettlingMax': 1.1826717168233585,
'Overshoot': 29.748622946474416,
'Undershoot': 0.3859422958870473,
'Peak': 1.1826717168233585,
'PeakTime': 0.13430991466739592,
'SteadyStateValue': 0.9999984629917944}
@bnavigator
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Thanks for bringing up the issue @juanodecc. I can confirm, although the plot is smoother with the new timevector calculation in 0.8.4 (#420), that the Overshoot value is still not what you would expect.

Figure_1

python-control/control/timeresp.py

Line 817 in 8cba949

'Overshoot': 100. * (yout.max() - InfValue) / (InfValue - yout[0]),

yout[0] here is 0.386 instead of 0. Not sure, why this is referenced here. The step_response() of a system without any further specification of initial values should start from default initial value of 0.

@sawyerbfuller, do you think the response vector calculation should be adjusted or just the calculation of Overshoot?

@sawyerbfuller
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This is a good catch. out[0] is not zero because this is not a strictly proper transfer function: the order of the numerator and denominator are the same.

My take on it is that the overshoot calculation has a bug that neglects this case. It should not depend on the value of yout[0].

@sawyerbfuller sawyerbfuller linked a pull request Mar 4, 2021 that will close this issue
fixed bug in calculation of percent overshoot #555
Merged
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fixed bug in calculation of percent overshoot sawyerbfuller/python-control
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@bnavigator @juanodecc @sawyerbfuller